A replacement cylinder can look almost identical to the old one and still be the wrong fit. When you need to know how to measure cylinder lock dimensions, the key is measuring from the centre fixing point, not from end to end. Get this right and you can order a compatible lock that sits neatly within your door furniture and provides the security level you expect.
This guide applies mainly to Euro profile cylinders, the common type used on uPVC, composite, aluminium and many timber doors. These are available as key/key, key/thumbturn, half cylinders and in a wide range of sizes.
What cylinder lock size means
A Euro cylinder is measured in two halves from its centre. The centre is the position of the fixing screw and cam, not necessarily the physical middle of the lock.
For example, a cylinder described as 35/45mm measures 35mm from the centre to one end and 45mm from the centre to the other. Its overall length is 80mm, but the individual measurements matter because most doors are not the same thickness on both sides once handles, escutcheons or security furniture are included.
The first figure is normally the outside measurement, with the second figure for the inside. However, product conventions can vary, so always check the supplier's stated orientation. More importantly, make a note of which side of your existing cylinder faces outside before removing it.
How to measure a cylinder lock while it is fitted
The most reliable method is to remove the cylinder, but an initial measurement can be taken while it remains in the door. This is useful when you are checking whether your current lock is likely to be too short or too long.
Open the door and look at its edge. Find the long fixing screw in line with the cylinder, usually beneath the latch or central locking mechanism. This screw marks the cylinder centre.
Measure from the centre of that screw to the outer face of the external handle, escutcheon or door furniture. Then measure from the screw centre to the inner face of the internal furniture. Do not measure only the visible part of the cylinder. The handle backplates may add several millimetres on each side, and they must be included in your calculation.
This gives you the approximate outside and inside cylinder lengths needed. If the door is fitted with a security handle, measure to the surface where the cylinder will finish inside the handle aperture rather than to the front of the handle itself.
An in-door measurement is a good starting point, but it can be affected by angled views, recessed furniture and tape-measure positioning. Remove the lock before ordering if possible, particularly where an exact replacement is needed for a landlord, business premises or a high-security upgrade.
Remove the cylinder for an exact measurement
Removing a standard Euro cylinder is straightforward provided the door is open. You will usually need only a screwdriver and the correct working key.
First, open the door fully. Remove the long central fixing screw from the door edge. Insert the key and turn it very slightly, usually around 10 to 15 degrees, until the cam lines up with the cylinder body. The cam is the rotating central piece that operates the lock mechanism. Pull the key gently towards you and the cylinder should slide out.
Do not force it. If it will not move, adjust the key by a fraction in either direction until the cam aligns. On a thumbturn cylinder, turn the thumbturn slightly instead. If the lock is damaged, seized or you do not have a working key, avoid trying to drill or force it out. A locksmith can remove and replace it without causing unnecessary damage to the door or multipoint mechanism.
Place the removed cylinder on a flat surface. Measure from the centre of the fixing-screw hole to each end of the cylinder body. Record both measurements in millimetres, such as 30/35mm, 40/40mm or 45/55mm.
Check the cylinder type before you order
Size alone does not identify the correct replacement. Check whether your cylinder is key/key, where a key operates both sides; key/thumbturn, where the inside has a turn knob; or a half cylinder, which has key access from one side only. Half cylinders are often used on garage doors, cupboards, gates and some commercial applications.
Also check the profile. A standard Euro profile has the familiar rounded shape with a narrow central waist. Oval cylinders, rim cylinders and some specialist commercial cylinders use different measurements and are not interchangeable with a Euro cylinder just because the overall length appears similar.
Choose the correct inside and outside lengths
The cylinder should sit as close to flush with the outer door furniture as practical. A small amount of projection may be unavoidable, but a cylinder that extends significantly beyond the handle or escutcheon can be easier for an intruder to grip and attack.
As a general guide, aim for the external end to project no more than around 3mm beyond the security furniture. This depends on the handle design and cylinder type, so treat it as a sensible target rather than an absolute rule. A cylinder fitted too deeply can also be awkward to use, especially with thick protective escutcheons.
The internal side needs similar care. A cylinder that is too short may sit recessed behind the handle and make key use difficult. One that is excessively long may look untidy or catch on clothing, although internal projection is usually less of a security concern than external projection.
Door thickness is only part of the calculation. Include the thickness of both handle backplates, escutcheons, decorative plates and any door-mounted security furniture. A 44mm composite door with 10mm of external furniture and 8mm of internal furniture, for example, may need a different cylinder size from a 44mm timber door with slim handles.
Do not overlook the cam and locking function
Most replacement Euro cylinders for domestic multipoint locks use a standard 10mm cam. This is the central component that engages with the lock case when the key turns. Although standard cams are common, some specialist locks use alternative cam styles or dimensions. If you are replacing a cylinder in a commercial door, a restricted system or an older installation, compare the existing cam carefully or seek advice before ordering.
Think about how the door is used as well. A key/thumbturn cylinder can make escape quicker from inside because no key is needed to operate it. It may be a practical choice for a main entrance, provided the door is not close enough to a letterbox, glazed panel or other opening that could allow someone to reach the thumbturn.
A key/key cylinder may be better where that risk exists, but occupants should know where the internal key is kept in an emergency. The right choice depends on the layout of the property, who uses the door and the security requirements of your insurer or managing agent.
Match security level to the door
Measuring correctly prevents a fitting problem. Choosing the right security specification helps protect the property after fitting. For external doors, consider a cylinder with recognised anti-snap protection, especially on uPVC and composite doors with multipoint locking systems.
Look for cylinders tested to TS 007 3 Star or Sold Secure Diamond where appropriate. A 3 Star cylinder is designed to provide attack resistance without relying on a separate security handle for its rating. Alternatively, a compatible 1 Star cylinder may be paired with 2 Star security furniture, but both parts need to work together as intended.
For internal doors, sheds, garages or low-risk applications, a basic cylinder may be suitable. The trade-off is straightforward: higher-security cylinders generally cost more, but can offer stronger resistance to snapping, drilling, picking and bumping. For a front or rear entrance, this is usually a worthwhile place to invest.
Common cylinder measuring mistakes
The most frequent error is ordering by total length alone. A 35/45mm cylinder and a 40/40mm cylinder are both 80mm overall, yet they position the cam differently and will not sit correctly in the same door.
Another mistake is measuring the old cylinder end to end without allowing for fitted furniture. This often results in a replacement that protrudes outside. It is also easy to reverse an offset cylinder when ordering. Label your notes clearly as outside and inside, and take a photograph of the cylinder in the door before removal if there is any doubt.
Finally, do not assume every cylinder uses the same key arrangement. If you need several doors to work from one key, choose a suitable key-alike option from the outset. If the lock is part of a master-keyed or restricted system, replacing one cylinder independently may affect access arrangements across the building.
A careful measurement takes only a few minutes, but it can prevent the inconvenience of ordering twice or leaving a door less secure than it should be. If the lock type, size or security upgrade is unclear, keep the measurements and a clear photograph to hand, then speak to a locksmith expert before you buy. Secure Choice can help you identify the right cylinder and, where needed, arrange professional fitting through a locksmith network.